1. Field of the Invention
The present invention relates to a cage for rolling bearing for use in general industrial machine, automobile engine, machine tool, steel machine, etc. More particularly, the present invention relates to improvements in the function of a press cage made of steel.
2. Description of the Related Art
As a cage for rolling bearing, there has heretofore been occasionally used a press cage obtained by subjecting a cold-rolled steel plate SPCC or SPCE material to pure nitriding (NH3 gas nitriding) or soft nitriding such as gas soft nitriding, toughtriding and ionic nitriding.
When a cage is subjected to nitriding, the matrix of the cage is hardened to obtain a surface hardness Hv of from 350 to 600 resulting in an excellent abrasion resistance. Further, since the nitride compound formed on the surface of the matrix has some resistance to high temperature softening, the adhesion or welding of the cage to the inner or outer race or the rolling elements can hardly occur, thereby providing good seizing resistance. Moreover, since the nitride compound is chemically very stable and inert, the cage exhibits an excellent corrosion resistance in a corrosive atmosphere. Further, nitriding is effected at a temperature as low as not higher than A1 transformation point (723xc2x0 C.), i.e., 400xc2x0 C. to 600xc2x0 C., causes little heat treatment strain and thus exerts a sufficient effect on parts which can be easily deformed due to its small thickness such as cage.
In recent years, however, rolling bearings have been subject to severer working conditions. In particular, when the lubricant film runs out, even a nitrided cage can easily wear. As the bearing is operated, the resulting abrasion dust causes the generation of dents on the inner and outer race or rolling elements. Further, the production of abrasion dust and the exhaustion of lubricant are accelerated, thereby causing seizing on the cage.
As a means for solving the foregoing problem there is proposed in JP-A-10-147855 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) a technique which comprises forming on the surface of a compound layer formed by nitriding on iron-based mechanism parts a porous layer having a thickness of from 1 to 20 xcexcm which is from 10 to 50% of that of the compound layer wherein the porous layer is impregnated with a lubricant by vacuum impregnation or heat impregnation to improve the lubricating properties thereof.
In the foregoing conventional technique, however, the thickness of the compound layer is only relatively defined. Therefore, the upper and lower limits of the thickness of the compound layer are determined by the thickness of the porous layer. Accordingly, the thickness of the compound layer is occasionally very great. In this case, the surface hardness of the entire compound layer is reduced, making it difficult to secure desired abrasion resistance or deteriorating dimensional accuracy. Further, the foregoing conventional technique is disadvantageous in that the step of impregnating the porous layer with a lubricant by vacuum impregnation or heat impregnation after nitriding adds to the production cost.
It is an object of the present invention to provide a cage for rolling bearing having assured stable abrasion resistance at a reduced cost by predetermining the tolerance of the thickness of the porous layer and porous layer in a compound layer formed by nitriding.
The inventors made extensive studies. As a result, it was found that the abrasion resistance and seizing resistance of a cage for rolling bearing which has been subjected to nitriding such as pure nitriding and soft nitriding depend on the thickness of the dense layer and the porous layer in the compound layer, respectively. Thus, the present invention has been proposed.
In other words, a cage which solves the foregoing problems can be provided by subjecting a press cage made of cold-rolled steel plate to nitriding such as pure nitriding and soft nitriding so that a dense layer has a thickness of from 3 xcexcm to 20 xcexcm and a porous layer has a thickness of from 2 xcexcm to 25 xcexcm, respectively.
The term xe2x80x9ccompound layerxe2x80x9d as used herein is meant to indicate a layer made of a nitrogen compound integrated with an iron-based substrate. The term xe2x80x9cdense layerxe2x80x9d as used herein is meant to indicate a layer disposed on the iron-based substrate in the nitrogen compound layer. The term xe2x80x9cporous layerxe2x80x9d as used herein is meant to indicate a layer having a porosity of from 10% to 60% disposed on the side of the dense layer opposite the iron-based substrate in the nitrogen compound layer.